The present invention relates to a frequency interpolation apparatus and method for obtaining through synthesis approximate values of the frequency components of a given signal removed or suppressed in a specific frequency band, and interpolating the approximate values to the given signal to thereby improve spectrum distribution.
2. Description of the Related Art
Music data is widely distributed recently in the form of MP3 (MPEG1 audio layer 3), FM (frequency modulation) broadcasting, television sound multiplex broadcasting and the like. In order to avoid an increase in data amount to be cased by an excessively broad bandwidth and prevent an occupied bandwidth from being broadened, the high frequency components of a subject audio signal or the like are suppressed to lower the upper limit frequency. For example, if the frequency components of 15 kHz and higher of an audio signal having an upper limit frequency of 20 kHz are suppressed to lower the upper limit frequency, the sampling frequency can be lowered by ¾ so that the data rate can be lowered. However, it is obvious that the sound quality is degraded as compared to that of the original signal. It has been studied to approximately recover the suppressed frequency components. One approach to frequency components recovery is to distort a subject signal, extract the frequency components to be used for suppressed bandwidth interpolation from the distorted signal by using a filter, and add the extracted frequency components to the subject signal to recover the original signal.
Another approach is to extract sound components having combinations of fundamental and harmonic components from an original audio signal, predict the harmonic components higher than the band of the original audio signal from the extracted sound components, and exterpolate the predicted harmonic components to the original audio signal.
According to the former approach, the waveform of an audio signal is distorted by using a limiter circuit or the like to generate harmonics so that it is not certain whether the harmonics can approximate those contained in the original audio signal.
If the latter approach is applied to an original audio signal obtained by limiting the band of an original voice or the like, it is not possible for tone color components of pure sound to predict the harmonic components and exterpolate them. Similarly, it is not possible for sound components with the harmonic components being removed by band limitation to predict the removed harmonic components and exterpolate them.
According to another approach developed recently, a subject signal is frequency-analyzed to estimate the residual spectrum pattern, intensity and the like of suppressed frequency components from the analyzed spectrum pattern, synthesize the estimated parameters and add them to the subject signal. Although this approach is excellent in terms of sound quality improvement, there is some practical problem. Namely this approach requires a great amount of calculations in digital signal processing, because it requires Fourier transform and inverse Fourier transform of high resolution and short time over the board band of the subject main signal.
This results in excessive requirements of the arithmetic operation rate and circuit scale of a digital signal processor (DSP) and hence lowered practical value.
The present invention has been made under the above-described circumstances and aims to provide a signal interpolating apparatus and method for recovering an original signal at a high quality from an audio signal with a suppressed specific frequency band, and a signal interpolating apparatus and method for efficiently recovering a signal approximate to the original signal even with a relatively small arithmetic operation scale.